Portable timing analyzers are typically used to test the ignition timing of an engine. A timing analyzer typically comprises a simple hand held, gunshaped instrument having a xenon strobe lamp incorporated therein. The analyzer is connected to a spark plug lead from the engine and is adapted to supply a pulse of light when a pre-selected current level in the lead is sensed.
Ignition timing refers to when a spark occurs in the spark plug gap. In a typical internal combustion engine, a set of breaker points alternating opens and closes a primary circuit between the battery, the primary winding of the ignition coil and ground. When the points are closed, the primary circuit is complete and current flows through the circuit. When the points are opened, a collapsing electro-magnetic field around the primary circuit produces a collapsing magnetic field in the adjacent secondary circuit, which is connected to a spark plug. The changing electro-magnetic field in turn produces an increasing voltage in the secondary circuit. When a sufficient voltage is present across the gap in the spark plug, the spark plug fires, whereby a fuel charge in the combustion chamber of the piston is ignited, and a current spike appears in the spark plug lead.
The rapid ignition of the fuel charge in the cylinder forces the piston downward on a power stroke. The piston is mechanically coupled to the crankshaft, which is thereby caused to turn. The fuel charge requires a short period of time to ignite and reach its full power. Consequently, the fuel charge is generally ignited a few degrees before the piston reaches top dead center ("TDC") of the cylinder, so that the burning charge properly forces the piston downward at the point of maximum efficiency.
In order to adjust the spark plug ignition timing so that the spark plug fires a specified number of degrees before the piston reaches TDC, timing marks are generally included on a stationary part of the engine, and a single mark is generally included on the rotating fly wheel. As the crankshaft spins, the fly wheel spins with it. The timing analyzer, which is in electrical contact with the spark plug lead, is triggered each time the spark plug fires, and produces a pulse of light. When the timing analyzer is aimed at the timing marks on the engine, the strobe effect of the pulses of light permits service personnel to align the timing marks on the engine with the rotating mark on the flywheel to establish proper ignition timing. When the engine is properly timed, the mark on the fly wheel lines up with the appropriate timing mark on the engine block.
In an internal combustion engine, and in particular a conventional four stroke engine, the piston completes four strokes within the cylinder during each operating cycle of the engine. For example, a four stroke engine has (1) an intake stroke, (2) a compression stroke, (3) a power stroke, and (4) an exhaust stroke.
On the intake stroke of the four stroke engine, the piston moves toward the bottom of the cylinder and creates a vacuum above it in the cylinder head. A camshaft mechanically coupled to the crankshaft causes an intake valve on the head of the cylinder to open and an exhaust valve to close. The intake valve delivers a air fuel mixture to the cylinder. When the piston begins to move upward in the cylinder during the compression stroke, the intake valve closes and the air fuel mixture is compressed. When the piston nears the upper end the cylinder, the spark plug fires and ignites the mixture. The rapid burning of the fuel forces the piston downward during the power stroke. At the bottom of the power stroke, the exhaust port opens and the exhaust gas flows out the port, assisted by the upwardly moving piston during the exhaust stroke.
Later model engines have a variation of a conventional four stroke engine, which is referred to as a distributorless ignition system ("DIS"). In the DIS four stroke engine, the engine has a series of double ended coils, where each coil fires two spark plugs simultaneously. Each coil is coupled through an ignition module to a timing circuit, which is generally included within an on-board computer. The timing circuit, through the ignition module, produces a current spike in the spark plug lead in much the same way as the set of breaker points produces a current spike in the conventional four stroke engine, however, the current spike is more controlled, both in duration and intensity.
In the DIS four stroke engine, the first spark plug on the coil fires normally in a first cylinder that is on a compression stroke to ignite the fuel charge, while the second spark plug fires a "waste spark" in a second cylinder that is on an exhaust stroke, which does not ignite a fuel charge. Each spark plug lead therefore experiences two current spikes for every cycle--one current spike for the power stroke and one current spike for the exhaust stroke.
Moreover, the ignition system of the DIS four stroke engine is more efficient than the ignition system of the conventional four stroke engine. The amplitude and duration of the current in the primary windings of the ignition coils are closely controlled in the DIS four stroke engines. Consequently, the current needed to fire each spark plug in the DIS engine is reduced.
In a conventional four stroke engine, the lead from the spark pug carries the large current spike from the spark plug firing and various secondary current spikes caused form "noise" in the system--typically noise caused by other cylinders. Consequently, the trigger in the timing analyzer must be set to a relatively high level for a conventional four stroke engine to trigger only during the actual spark plug ignition.
However, in the more advanced DIS four stroke engine, the current spikes from the compression stroke and the exhaust stroke are relatively smaller. Additionally, there is less noise through the spark plug lead than in a conventional four stroke engine. Consequently, if a conventional four stroke timing analyzer is used on a DIS four stroke engine, the timing analyzer may not operate correctly because the current spikes in the DIS four stroke engine might not be high enough to trigger the analyzer.
Moreover, service personnel are typically trained to visually check the rate of light pulses emanating from the timing analyzer before testing ignition timing to roughly determine if the analyzer is operating properly. If a conventional four stroke timing analyzer is used on a DIS four stroke engine, the service personnel would observe twice the rate of light pulses emanating from the timing analyzer, since the spark plugs in a DIS engine fire twice as fast as a conventional four stroke engine. Consequently, service personnel can become confused by the increased rate of light pulses and can believe that the timing analyzer is not operating properly.
Prior art timing analyzers have so far failed to overcome the aforementioned problems. It is known in the timing analyzer art to provide analyzers with a two position switch, whereby in one position, the switch slightly increases the sensitivity of the timing analyzer by lowering the trigger threshold, and in the other position, the switch slightly decreases the sensitivity of the timing analyzer by increasing the trigger threshold. However, these prior art timing analyzers are primarily designed to compensate for different current trigger levels in conventional four stroke engines, and are not designed for the considerably lower current threshold in the DIS four stroke engine. Moreover, the prior art timing analyzers do not allow flexibility in setting the trigger to properly respond to different current threshold levels, so as to manually select a desired usable threshold level.